Prior art of optical element changing devices are so called filter wheels comprise a set of optical filters mounted on a disk which moves them into the light path of attached optical devices by revolving. In order to achieve this, the prior art filter wheel's rotational axis is usually parallel but decentered (offset in x-y-plane) in respect to the path of light (z-axis) of the surrounding optical system, the filter wheel is embedded in (FIG. 1). An example of a prior art filter wheel can be found in U.S. Pat. No. 6,567,225 B1.
There are widely used optical systems which impose that the filter wheel must be placed in front of the telescope, thus causing obstruction of the incoming light. Prime-focus telescopes for instance are such systems. (FIG. 3).
Prior art filter wheel designs possess certain shortcomings for such applications as follows:                a) When installed in front of telescopes (FIG. 3a), prior art filter wheels cause substantial obstruction of the incoming light because of their proportionally large diameter depending on the number of housed filters. This results in a decreased image quality because of the reduction of the overall incoming light as well as less contrast and resolution of the image.        b) Moreover the obstruction caused is not centrosymmetric in respect to the telescope's optical axis (FIG. 3a) which reduces the image quality of the whole field of view by uneven deformation of the observed objects.        c) By not being centered in respect to the overall optical systems light path (z-axis), the center of gravity is also decentered which causes unsymmetrical mechanical stress of the surrounding support structures. When capturing data while the overall optical system (usually a telescope with attached filter wheel and image capture device) is moving (in respect to the surrounding field of gravity) in order to follow the object of interest, the uneven mechanical load causes a certain deflection of the support structures. This results in a deformation of the observed object's image.        d) Furthermore it is vital for direct-driven telescope mounts to be balanced in directions perpendicular to the field of gravity. With prior art filter wheels, this can only be achieved by using additional counterweights which further increases the overall mechanical load.        e) Prior art filter wheels need an opening or respectively an empty holder for optical elements for the passing through of light without any interaction with one or more of the housed optical elements.        f) Prior art so-called ‘centered filter wheels’ do not overcome these problems. These filter wheels use two overlapping filter wheels to produce a symmetric obstruction. But by not being circular they still cause an unsatisfactory high amount of light obstruction. Furthermore this design implies that every rotary disk must have au empty holder or respectively an opening which cannot be used for mounting optical elements ill order to enable the passing through of light without any interaction with one or more optical elements housed. An example of a prior art centered filter wheel is the ‘CenterLine Dual Color Filter Wheel’ by the company ‘Finger Lakes Instrumentation’. FIG. 2 depicts such a centered filter wheel.        
What is needed is a circular as well as centrosymmetric (also referred to as point-symmetric) filter changer design in respect to the light path of attached optical devices (z-axis), winch minimizes the overall obstruction per filter as well as the obstructing shape (this is ideally a perfect circle) as well as having a centered point of gravity.